The enthalpy of elements in their standard states is taken as zero. Hence the enthalpy of formation of a compound is

Consider following example ;Cs+O2g→CO2g,∆H=-393 kJ, ∆H=HP0-HR0; Since HR0 in this case is zero, ∆H=HP0=-393 kJ; Similarly for an endothermic process, ∆H=HP0=+ve ;